The present invention relates generally to electronic circuits and, more particularly, to a frequency doubler which uses adaptive biasing in order to maintain a fixed duty cycle.
A typical frequency doubler for use in high-speed digital applications employs two biased one shot circuits, each one responsive to a particular transition of the input signal to be doubled. The output signals from the two one shots are ORed to provide the frequency doubled signal. The bias current to the one shots controls the widths of their output signal pulses.
In order to provide a fixed duty cycle, desirably fifty percent, over a range of output frequencies, the biasing of the one shot circuits must be able to be dynamically altered. A low-pass filter coupled to the frequency doubled output signal measures its average voltage level, and generates a suitable bias current to the one shot circuits. Thus, as the frequency of the input signal to the doubler increases, the output signal "high" time increases as a percentage of the cycle time. This results in an increase in the voltage level from the low-pass filter, which increases the bias current to the one shot circuits, causing a decrease in their pulse widths until the desired duty cycle is achieved.
The above-described prior art frequency doubler provides satisfactory results within a relatively narrow range of operating conditions. However, for a typical device designed to provide an output signal having a frequency in the range of 100 to 200 MHz, the biasing scheme of the prior art may be inadequate. As an example, a comparison of worst case conditions (maximum frequency, high operating temperature, low supply voltage and minus-three sigma fabrication process variations) with best case conditions (minimum frequency, low operating temperature, high supply voltage and plus-three sigma fabrication process variations) results in a ten-to-one range of bias requirements. Such a range of biasing current cannot be controlled by the frequency doubler biasing scheme of the type described above as prior art.
In view of the foregoing, it is clear that there exists a need to develop an improved apparatus for controlling the bias currents to the one shot circuits of a frequency doubler over what is currently known in the art. Such an apparatus should provide a fixed output signal duty cycle over a wide range of operational, environmental and process variations.